St John's college Cambridge argues that fines do not affect all students equally Photograph: Alamy

Community service tasks such as cleaning toilets and digging trenches are the fairest way to punish misbehaving students, says a Cambridge University college.

St John's college Cambridge has defended its decision to make students carry out manual labour for bad behaviour – as reported by the Cambridge Student – saying such tasks are more evenhanded than the financial penalties imposed by other Cambridge colleges and UK universities.

"The dean's policy is to make such an order in preference to imposing a fine. The college recognises that the effect of a fine varies according to students' ability to pay it," St John's says.

Jamie Stern-Weiner, who graduated from Cambridge University this summer and was often hit with library fines, admits that there is sometimes a need to punish students. "Libraries need to have books returned so others can borrow them, and you need some incentive to get them returned. While I don't love paying fines, I hate not being able to find a book in the library that should be there."

But Pete Mercer, NUS vice-president, warns that university punishments should come only after adequate warnings.

"That institutions are racking up such huge sums in fines suggests that they are gratuitous, and are treating students as cash cows. It's vital that universities considering levying fines take student hardship into account."

"If money is collected through fines, it must be put back into helping students through things like hardship funds and library resources."

Rosalyn Old, president of Cambridge University's student union, says community service punishments must be also be proportionate: "We support efforts to ensure that the effect of punishments doesn't depend on students' different financial backgrounds. But university staff should respect the fact that students are adults and should be treated as such."